The present invention relates to synthetic polymeric compositions. In particular, the present invention relates to the preparation of polymer membranes and the use thereof in the separation of components of gas mixtures, the separation of ions in aqueous solutions, and similar applications.
There is an ever-increasing need for improved techniques for the efficient and rapid separation of the components of mixtures. In particular, techniques for the separation of components of gaseous mixtures have many significant technical applications. Oxygen separated from the air is utilized in medical applications and enrichment of combustion processes. Nitrogen is used to protect perishables and air-sensitive materials. The removal of carbon dioxide and hydrogen sulfide from natural gas reduces pollution; the carbon dioxide may further be used for tertiary oil recovery. Methane reclaimed from landfills and mines can provide useful fuel. With improved technology, exhaust gases from internal combustion engines could be separated and recycled.
Membrane-based separation systems in theory offer enormous potential savings in energy over standard techniques (for example, cryogenic processes used for separation of gases). During the past decade, a variety of different membrane systems have been introduced commercially. These have been made possible both by the synthesis of new polymeric materials and by the development of asymmetric membranes, in which a thin skin of polymer with high selectivity is grown on a porous structural polymer support. Although some progress has been made in such membrane technology, there has heretofore been found an inverse relationship between selectivity and permeability. This has significantly limited the range of potential applications for such membranes, and has focused attention on the search for techniques to control the porosity of such membranes.
There have been a number of proposals in the prior art of methods for controlling the porosity of polymeric membranes for use in, e.g., reverse osmosis or ultrafiltration methods. For example, U.S. Pat. No. 4,452,424 to Tweddle et al. describes preparation of polysulfone cast films which are partially gelled with mineral acid prior to or during a conventional water gelation procedure. By adjusting the concentration of acid, it is alleged that the porous structure of the resulting membrane may be controlled and improved.
U.S. Pat. Nos. 4,717,393 and 4,717,394 to Hayes describe the preparation of crosslinked polyimide gas separation membranes which exhibit superior selectivity relative to uncrosslinked polyimides. The selective permeabilities observed are attributed to optimization of the molecular free volume in the polymer.
U.S. Pat. No. 4,761,233 to Linder et al. describes the casting of membranes from solutions comprising mixtures of at least one pore-forming polymer and at least one film-forming polymer. The pore-forming polymer is selected as one which if cast alone would contract to form either large pores or a non-uniform distribution of material.
While these materials have some utility with respect to separation of components of some types of mixtures, they have heretofore been inadequate for many desired separations, in particular for the separation of components of gaseous mixtures. It is known that the molecules of different gases may be differentiated on the basis of their kinetic diameters, which are calculated from measurements of the thermodynamic properties of each gas. For example, the following values for kinetic diameters (in Angstroms) have been reported: He=2.6; H.sub.2 =2.89; Ne=2.75; Ar=3.40; O.sub.2 =3.46; N.sub.2 =3.64; CO.sub.2 =3.3; and CH.sub.4 =3.8. In theory, any two gas molecules having different kinetic diameters could be separated on the basis of this difference in size.
In practice, however, achievement of this goal has been difficult at best, particularly when the difference in kinetic diameters is relatively small. Thus, there is a need for membranes with higher separation factors, i.e., ratios of the permeability of one gas to another through the membrane. For example, the best separation factors reported in the literature for mixtures comprising O.sub.2 /N.sub.2 and CO.sub.2 /CH.sub.4 are 16 and 60, respectively.
Through the use of known techniques, it has heretofore been possible to vary the density of polymer films over a fairly broad range. In particular., various polymers possessing pi conjugation, such as the polyaniline family of polymers, may be cast from solution or hot-pressed into fully dense films or otherwise processed into hollow fibers or asymmetric membranes.
It is also known that certain polymer systems which are electrically insulating as formed may be modified subsequent to their preparation by acid doping, or by chemical or electrochemical n- or p-doping to render the polymers electrically conductive. In this context, by dopant is meant a guest species which maintains electrical neutrality in the host polymer, while altering the pi electron density of the polymer and/or its morphology. The dopant species forms coordinate covalent and/or ionic bonds.
Thus, U.S. Pat. Nos. 4,851,487 (Yaniger et al.), 4,806,271 (Yaniger et al.), 4,798,685 (Yaniger) and 4,822,638 (Yaniger) describe application of a covalent doping agent to a base-type non-conductive polymer substrate containing carbon-nitrogen linkages to convert the substrate to an electrically conductive polymer. The products are described as useful in the preparation of electronic devices.
U.S. Pat. No. 4,615,829 to Tamura et al. describes an electroconductive organic polymer containing an electron acceptor as a dopant and consisting essentially of a linear polymer having as a main repeating unit a quinonediimine structure. According to Tamura et al., this polymer is prepared by oxidative or electro-oxidative polymerization of an aniline compound or a water-soluble salt thereof in a reaction medium containing a protonic acid. The product is a conductive polymer which exhibits stability without an additional doping step, because it allegedly has already undergone doping during the course of the oxidative polymerization. Tamura et al. suggests that the polymer may be chemically compensated with ammonia, whereby it undergoes significant loss of electroconductivity; if this polymer is then doped with an electron acceptor such as sulfuric acid, the original high electro-conductivity is restored. There is described a process wherein the polymer is produced by first reducing the electroconductive organic polymer with a reducing agent and then oxidizing and simultaneously doping the reduced polymer with an oxidizing agent which is effective as an electron acceptor; in this manner, the dopant may be replaced by a different dopant such as a halogen or Lewis acid. While no particular utility for the electroconductive polymers of Tamura et al. is disclosed, it is apparent from the specification that the salient feature of the subject materials is an electroconductivity of not less than 10.sup.-6 S/cm.
In order to provide improved separation membranes, it has been proposed to treat preformed porous or microporous membranes of non-conducting polymers with monomeric precursors of conducting polymers; the monomers are then polymerized in situ so as to plug holes in the preformed membrane (a technique generally known in the art as "healing" the preformed membrane). Membranes prepared in this manner are described in, e.g., Japanese Published Applications Showa 62-110729 (Matsushita) and Showa 63-270505 (Tokuyama Soutatsu K. K.). In addition, electrically conductive membranes have been prepared by solution coating of microporous polypropylene or polytetrafluoroethylene with the emeraldine base of polyaniline in dimethylformamide or dimethylsulfoxide soution [Loh, I-H. et al., Journal of Membrane Science 50:31 (1990)]. Central to these methods is the use of porous or microporous membranes as starting materials. Although some positive modifications in permeabilities and/or separation factors are achieved by these techniques, it is not possible to compensate entirely for the extremely high porosities of the preformed membranes and the final products are far from theoretical full density.
Japanese Published Application Heisei 2-21560 (Tokuyama Soutatsu K. K.) describes films wherein electrically conducting polymers are uniformly dispersed with non-conducting polymers. These films, however, generally have a porosity in the range of 10 to about 95%; for some applications, the porosity is as high as 98%. The exemplary products are described as having porosities in the range of 58-92%. Thus, the disclosed membranes are also quite far from theoretical full density.
Japanese Published Application Showa 64-38125 (Kao K. K.) describes gas permselective membranes composed of conjugated polymer compounds, wherein the conjugated polymer compounds are obtained by electrochemical oxidation polymerization of monomers in a solution containing supporting electrolytes. The electrolytic oxidation polymerization is performed by applying a voltage between electrodes immersed in a solution of monomers dissolved or dispersed in a polar solvent; electrolytes used in common electrolytic reactions can be used as the supporting electrolytes. Anodic doping occurs during the electrolytic oxidation polymerization, whereby the counterions of the supporting electrolytes in the electrolytic solution are captured by the polymers. The captured dopants may remain in the conjugated polymers of the invention, or the polymers may be undoped.
While the electropolymerized films prepared in this manner may have some utility for use as separation membranes, they are far from optimal for several reasons. Almost all electropolymerized films are highly porous (&gt;90%), and therefore membranes prepared from such films would for the most part not perform adequately even after heavy doping. In view of their manner of preparation, moreover, some amount of dopant is usually buried in the polymer matrix and/or covalently bound to the polymer; this dopant cannot readily be removed from the polymer matrix. As a consequence, the electropolymerized films have a different morphology from essentially dopant-free polymer films which are, e.g., solution cast. In particular, in electropolymerized films the presence of the dopant during polymerization causes the polymer chains to pack farther apart initially than would be the case when films are prepared in the absence of dopant. This leads to a much broader (and clearly less desirable) distribution of free volume. Upon removal of some of the dopant, the ability to separate components of similar size and shape from a mixture thereof would not be improved; in fact, it has been observed that the separation factor actually decreases for, e.g., a gas pair like oxygen and nitrogen.
It is an object of the present invention to provide membranes for use in the separation of the components of various mixtures, and in particular for the separation of the components of gaseous mixtures.
It is another object of the present invention to provide method for treatment of an as-synthesized polymer to improve its utility as a membrane for use in the separation of components of various mixtures, in particular mixtures of gases.
Another object of the present invention is the selective modification of conjugated fully dense polymer membranes, such as polyaniline, for specific and selective gas separations, thereby enabling selection of one species over others in a mixture.
Another object of the present invention is to enable control of the separation of components of a mixture using a polymer membrane which may be modified by in situ treatment methods.
Still another object of the invention is to provide conjugated polymer/nonconjugated polymer or conjugated/semiconjugated fully dense polymer blends, copolymers, and polymer alloys.
Yet another object of the invention is to enable control of the average pore size of the polymer membrane.